IT was a 6.9 billion kilometre long shot, fired blind at a tiny target. Now the Rosetta space probe must find a spot to place its sensors before this comet erupts in fury.

Rosetta’s chase has not been an easy one: It’s had to hit the exact spot at exactly the right time repeatedly in its slingshot-trail around the solar system. Not to mention it was flying blind in a desperate bid to conserve enough battery power for some dicey upcoming manouvres.

Now all the maths has paid off.

But there is still a race against time to rendezvous with the space-rock’s surface before it is warmed enough by the approaching Sun to start jetting a tail of vapour out into space.

More than 400 million kilometres from its March 2004 launch site, the Rosetta will meet its prey, Comet 67P/Churyumov-Gerasimenko (C-G), later this afternoon.

Both are passing through the asteroid belt, between the orbits of Mars and Jupiter.

Where is Rosetta?

The three-tonne craft should tonight come to “rest” about 100km from the comet as both move through space.

“It’s taken more than 10 years to get here,” spacecraft operations manager Sylvain Lodiot said.

“Now we have to learn how to dock with the comet, and stay with it for the months ahead.” Astrophysicists believe comets are clusters of the oldest dust and ice in the Solar System, the rubble left from the formation of the planets 4.6 billion years ago.

Getting there won’t be easy.

Target in sight ... Rosetta took this photo of 67P/Churyumov-Gerasimenko from a distance of 234 kilometres on August 4, two days before its attempt to enter orbit. Picture: ESASource: Supplied

Not only must Rosetta’s Earth-based engineers engage in some fine driving, the probe itself has to survive any unexpected “space weather”.

While the cosmic dust emitted by the comet may be scant at this distance from the Sun, any collision with a cloud of them could mean the end of the mission. But Rosetta has been built encased in a thermal foil shield and its vulnerable fuel lines protectively encased in the body of the probe for added protection.

And such clouds will get all too common in the next few months as the comet continues its dive towards the Sun.

The final rendezvous manoeuvre will begin at about 6.30pm EST. An hour later the final approach starts, Rosetta will fire its thrusters for exactly 6 minutes and 26 seconds — helping to slow the probe and sling it into a close orbit around the comet.

ESA engineers will have to wait 22.5 minutes to know whether or not the manoeuvre was successful: That’s how long it will take the radio transmissions to reach Earth from the probe.

To keep in this orbit, Rosetta will have to fire its thrusters every three or four days.

The first few weeks will be a get-to-know exercise, as the spacecraft traverses elongated circuits of the comet, scanning its surface.

Rosetta has already been returning images of the comet’s mysterious core. Much more detailed images are expected in the coming hours and days.

On November 11, the plan is for Rosetta to approach within a few kilometres of the comet to send down a 100kg refrigerator-sized robot laboratory, Philae (It was named after the Island where the Rosetta stone was discovered, an inscription which helped decode Egyptian hieroglyphs).

The probe, which will have to avoid ice crystals and dust particles, will soon begin seeking a suitable landing site for Philae. Last month as Rosetta closed in, its cameras revealed that C-G resembled a duck — two lobes, one big and the other small, connected by a “neck”.

“That was a bit of a surprise,” said Philippe Lamy of the Astrophysics Laboratory in Marseilles.

A matter of scale ... 400,000,000km from Earth, the 32m Rosetta space probe — at a distance of 10km from Comet 67P/C-G — will soon land a 1m square landing probe, Philae”, on the compet’s surface. Picture: ESASource: Supplied

“Several theories have already been aired to explain this shape, but the likeliest in my book is that it came from two bodies which fused while the Solar System was being formed.”

The unexpected shape will limit the choice of landing site, Lamy said.

“You can reasonably argue that it will impose additional constraints.”

The comet itself has very little gravity: In order to remain on its surface, the lander will have to fire harpoons into the surface to act as anchors.

Exactly what that surface will be is unknown: It may be something akin to permafrost, it may be solid ice and stone.

The Philae lander will carry out experiments in chemistry and texture for up to six months.

After Philae expires, Rosetta will accompany C-G as it passes around the Sun and heads out towards the orbit of Jupiter.

Tough touchdown ... Rosetta’s lander, Philae, will find it difficult to find a smooth space to land on the comet’s surface. Pictures: ESASource: Supplied

COMETS THE KEY TO LIFE?

These so-called dirty snowballs could be the key to understanding how the planets coalesced after the Sun flared into life.

One theory is that by bombarding the fledgling Earth with water and organic molecules comets helped kickstart life.

Until now, exploration of comets mainly entailed photographs taken by probes thousands of kilometres away on unrelated missions. Exceptions were the US probe Stardust, which brought home dust snatched from a comet’s wake, while Europe’s Giotto ventured within 200km of a comet’s surface.

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